The present invention relates to a preheating apparatus for Diesel engines, which is designed to ensure a rapid glow plug temperature rise characteristic.
In the past, certain types of Diesel engines, known as pre-combustion chamber engines, have been mostly equipped with glow plugs such that during the engine starting the glow plugs are heated red hot and the fuel is burned by contacting it with the glow plugs to thereby facilitate the starting of the engine, and there has been a desire that the time required for heating the glow plugs red hot be reduced as far as possible. In particular, passenger cars equipped with Diesel engines have recently been used for a variety of purposes due to the improved fuel economy of the Diesel engines. However, the vehicles equipped with Diesel engines have been found inferior in starting performance to those equipped with gasoline engines, thus making the necessity of preheating the Diesel engine particularly important in the case of passenger cars.
On the other hand, in order to start the Diesel engine smoothly, it has been necessary to preheat the glow plugs to a temperature of 700.degree. to 1000.degree.C., and in the past the time required for attaining such temperature (hereinafter referred to as a "preheating time") has been considerably long.
In view of these background circumstances, systems have been proposed in which as shown, for example, in Japanese Utility Model Publication No. 48262/77, a resistance unit having a very high positive temperature coefficient of resistance, such as, a resistance unit made from molybdenum disilicide is connected in series with a parallel circuit of glow plugs so that during the starting period a large current is supplied to the glow plugs through the action of the resistance unit and the temperature of the glow plugs is rapidly increased.
However, the prior art apparatus of this type has the following disadvantages.
(1) The resistance value of the resistance unit linearly increases with temperature so that the ratio of change in resistance between the lower and higher temperature ranges is about 6 times and consequently the resistance value at the normal temperature becomes 1/6 of the resistance value in the higher temperature range. Thus, the normal temperature resistance value cannot be set so small that the amount of starting current supplied to the glow plugs is limited considerably depending on the resistance unit and the resulting preheating time is 15 seconds as compared with the conventional time of 20 seconds showing only an improvement by 25%.
(2) When current is being supplied to the glow plugs, the current always flows to the molybdenum disilicide resistance unit so that the resistance unit always generates heat and it is always kept at elevated temperatures with the resulting large change of its characteristics with time. As a result, no matter how accurately the initial resistance value is set, the resistance value is subject to variation with the accumulated period of service time of the glow plugs, thus giving rise to such troubles as burnout failure, faulty heat generation, etc., of the glow plugs.